TED-Ed answers the question: what is dark matter and what does it have to do with stars?

Spiral galaxies are a stunning sight. As Don Lincoln explains in one of this week’s TED-Ed lessons, these galaxies were thought to rotate in highly predictable dances, with stars close to the center moving slowly and stars further away rotating quickly because they are pulled by the mass of the stars between them and the center. The stars located very far from the galaxy center were thought to rotate extremely slowly because of the reduced pull of gravity.

Only, that’s not how the stars on the edges of galaxies move at all. As Lincoln explains in this lesson, a crisis in astronomy occurred when it was discovered that these stars actually rotate much more quickly than expected. Scientists had to take another look at the Newton’s theories of gravity and mass to figure out why their predictions were so wrong.